The invention relates to a piezoelectric drive having an inner part and an outer part, wherein the inner part has a piezoelement and an oscillation element that cooperate to drive the outer part relative to the inner part.
Piezoelectric drives, that is to say drives by way of piezoceramic materials which may be mechanically changed by an electrical voltage field are suitable in particular for miniaturized applications, for example for motors with a motor volume of a magnitude of a few cubic centimeters or even with a volume which is smaller than one cubic centimeter. Further advantages of these drives are a high moment at low speeds, a simple controllability, a smooth running, a relatively simple construction and their insensitivity with regard to external magnetic fields as well as the fact that they themselves produce no magnetic fields.
Piezoelectric drives of the so-called standing-wave type as a driving element comprise at least one resonator which usually consists of a piezoelement and a resonance body (horn) mechanically coupled to the piezoelement, wherein the piezoelement and the resonator body are matched to one another and the piezoelement driven, in a manner such that the resonator oscillates in a standing wave. The horn comprises a tapering free end which points away from the piezoelement and which advantageously lies on a point of the greatest oscillation amplitude. It has been shown that such horn tips, if they are pressed in a suitable manner against a movable body, may drive the body in a directed manner, wherein the force transmission is essentially based on a friction fit.
Such a drive is described in “Piezoelectric Actuators and Ultrasonic motors” of Kenji Uchino (Kluwer Academic publishers, Boston, Dordrecht, London 1997). This drive comprises a pair of disk-like piezoelements arranged coaxially over one another which are operated polarized in opposite directions in a 3,3 mode. A horn connects essentially coaxially to the piezoelements. The horn tip is pressed against the surface of a body which is movably arranged parallel to this surface. It has been shown that the body may be driven in a directed manner with the help of the horn if the axis of the horn is not directed exactly perpendicularly towards the surface but forms a small, acute angle with the perpendiculars to the surface. If the resonator arranged in such a manner is operated at a resonant frequency, it drives the body in that direction in which the slightly oblique horn tip points. The induced, directed movement of the driven body is explained by the elliptical oscillations of the horn tip in a plane perpendicular to the surface of the body. A reversal of the movement direction is achieved by a re-orientation of the resonator axis.
A similar piezomotor is described in the publication DE-3920726 (Olympus Optical). In place of the resonator of the motor described briefly above, which is symmetrical relative to its axis, the motor according to DE-3920726 comprises an asymmetrical horn whose tapering end does not lie on the resonator axis. The resonator is arranged with the axis directed perpendicularly to the surface of the body to be driven, and with a suitable shaping of the horn tapering towards the horn tip, causes a directed movement of the body. At the same time, there are oscillation conditions at frequencies different from one another which produce movements in opposite directions. The movement direction may, thus, be set via the frequency driving the piezoelements. The drive is suggested for application as a linear drive or as a rotational drive, wherein the resonator axis is aligned perpendicular to an end-face of the rotor (axially) or perpendicular to the outer surface of the rotor (radially). The motor exhibits a large wear.
The piezoelectric drive according to DE-3920726 may be realized with relatively simple means as a rotation motor with an end-face drive. In the embodiment with the end-face drive it is also possible with simple means to mutually preload the rotor and resonator. It is particularly with regard to piezoelectric drives having an axially directed resonator axis that miniaturization limits arise, and one would like to go beyond these limits.
For motors which are to be very flat in the axial direction, it is therefore suggested (e.g. in EP-0505848, ETA SA) to use a centrally arranged, circular-disk-shaped piezoelement which may be driven in a planar mode. This piezoelement is coupled to a flat resonance body which is arranged coaxially to the piezoelement and which comprises a plurality of asymmetrical horn tips extending radially towards an outer ring. Driven by the piezolements, the horn tips again oscillate in elliptical movements by way of which the outer ring is rotatingly moved about the resonator in a directed manner. The described drive although being able to be designed very thin in the axial direction, is however always an inner runner. A preloading between the resonance body and the outer ring is not possible so that the drive reacts very sensitively to wear on the horn tips and the force able to be transmitted by friction remains limited.
The piezoelectric drives known from the state of the art are not suitable or hardly suitable for a drive of the type “outer runner” with which the inner part of the motor remains stationary and the outer part is driven. Conventional motors act primarily perpendicular to a surface to be driven, which entails much wear and a short serviceable life.
It is then the object of the invention to provide a piezoelectric drive of the type “outer runner” which consists of few parts, is simple to realize and is suitable for drives of different size and power.
This object is achieved by the piezoelectric drive as is defined by the present invention.